This invention relates generally to the fabrication of nanotube based electronic devices, and, more particularly, to the fabrication of electronic devices using nanotubes grown in a reaction chamber using a solvent based catalyst.
Carbon is one of the most important known elements and can be combined with oxygen, hydrogen, nitrogen and the like. Carbon has four known unique crystalline structures including diamond, graphite, fullerene and carbon nanotubes. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall or multi-wall, which can be obtained by rolling a sheet formed of a plurality of hexagons. The sheet is formed by combining each carbon atom thereof with three neighboring carbon atoms to form a helical tube. Carbon nanotubes typically have a diameter in the order of a fraction of a nanometer to a few hundred nanometers.
Carbon nanotubes can function as either a conductor, like metals, or a semiconductor, according to the rolled shape and the diameter of the helical tubes. With metallic nanotubes, it has been found that a one-dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic nanotubes can be used as ideal interconnects. Introducing a defect into a metallic tube can result in a single electron charging effect. The single electron charging effect can be used to make a single electron transistor. When semiconductor nanotubes are connected to two metal electrodes, the structure can function as a field effect transistor wherein the nanotubes can be switched from a conducting to an insulating state by applying a voltage to a gate electrode. Therefore, carbon nanotubes are potential building blocks for nanoelectronic devices because of their unique structural, physical, and chemical properties.
Existing methods for the production of nanotubes, including arc-discharge and laser ablation techniques, yield bulk materials with tangled nanotubes. The nanotubes in the bulk materials are mostly in bundled forms. These tangled nanotubes are extremely difficult to purify, isolate, manipulate, and use as discrete elements for making functional devices. Originally, carbon nanotubes produced by an arc discharge between two graphite rods was discovered and reported in an article entitled xe2x80x9cHelical Microtubules of Graphitic Carbonxe2x80x9d (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58) by Sumio Iijima. This technique is commonly used to produce carbon nanotubes, however, yield of pure carbon nanotubes with respect to the end product is only about 15%. Thus, a complicated purification process must be carried out for particular device applications.
Another conventional approach to produce carbon nanotubes, which was described in an article entitled xe2x80x9cEpitaxial Carbon Nanotube Film Self-organized by Sublimation Decomposition of Silicon Carbidexe2x80x9d (Appl. Phys. Lett. Vol. 71, pp. 2620, 1977), by Michiko Kusunoki, is to produce carbon nanotubes at high temperatures by irradiating a laser onto graphite or silicon carbide. In this case, the carbon nanotubes are produced from graphite at about 1200xc2x0 C. or more and from silicon carbide at about 1600xc2x0 C. to 1700xc2x0 C. However, this method also requires multiple stages of purification which increases the cost. In addition, this method has difficulties for large-device applications.
Some of the drawbacks of these two methods are that the tubes are formed under an extremely high temperature environment and are usually produced as bundles, embedded with catalyst particles which are covered with amorphous carbon. To fabricate devices using nanotubes produced from these methods, various cleaning and debundling steps are required. The debundled nanotubes are then suspended in a solution, which can then be positioned on a substrate with patterned electrodes or other circuitry. However, it is extremely difficult to control the placement and orientation of the nanotubes when using these methods. It is therefore very inefficient to fabricate electronic devices using nanotubes formed either by arc discharge or laser ablation.
U.S. Pat. No. 6,346,189 issued to Dai et al. on Feb. 12, 2002 discloses a method of selectively producing high quality single walled carbon nanotubes on a substrate using catalyst islands. The catalyst particles consisting of Fe2O3 or other transition metal oxides are suspended in methanol. According to the method, a first lithography step is used to pattern a substrate with catalyst islands, wherein the first lithography step uses e-beam lithography. Nanotubes are then grown using a chemical vapor deposition process. Electrical contact to the nanotubes is made by performing a second lithography step to form electrodes. However, during the second lithography step, the nanotubes may be damaged and contaminated.
Accordingly, it is an object of the present invention to provide a new and improved approach for fabricating nanotube based electronic devices.
To achieve the objects and advantages specified above and others, a method of fabricating a nanotube structure is disclosed which includes providing a substrate, providing a mask region positioned on the substrate, and patterning and etching through the mask region to form at least one trench. A conductive material layer is deposited within the at least one trench and a nanoparticle catalyst is deposited onto the conductive material layer within the at least one trench. The mask region is removed using a conventional lift-off technique, such as a single step lift-off process, to form nanoparticle catalyst coated electrodes. The nanotubes are formed from the catalyst using a reaction chamber with a hydrocarbon gas atmosphere.